One objective is to determine the lateral mobilities and lateral distributions of membrane-bound haptens in reconstituted lipid model membranes of defined composition, using the techniques of spin-label paramagnetic resonance, nuclear magnetic resonance, and freeze-fracture electron microscopy. Haptens of special interest are those that contain the paramagnetic spin label group as the haptenic moiety. A second objective is to determine whether or not there exists a causal, quantitative, and understandable relation between the lateral distributions, their lateral mobilities, and (a) the susceptibility of hapten-sensitized lipid vesicles to complement-mediated, and to lymphocyte-mediated, antibody-dependent attack, and (b) the degree to which such vesicles can elicit an immune response in vivo and in vitro.